Curse does not have the legal right to stick up for anyone in this case. This is a matter between Mojang and the modders.
As it stands, the only person whom the mod authors cannot reserve distribution rights against is Mojang itself. Mojang may, if it so chooses, share that right with others, but only if it so chooses. Until the company chooses to do so in an official capacity (i.e. something more like a Corporate Action than a random tweet from an employee), a mod author's rights are still explicitly reserved and acknowledged by the EULA and TOS.
Read Marc's quote in the thread on Minecraft Forums.
He basically confirmed Curse can police their forums how they see fit and the moderation teams have said they'll be keeping the same rules. So they will be shutting down mods that contain "stolen" content, regardless of EULA. As a private website they aren't bound by it and can completely choose to drop mods off the site for any reason they see fit.